Make elements/names.dart its own library

pkg/compiler/lib/src/js_backend/mirrors_data.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import '../closure.dart';
 import '../common.dart';
 import '../common_elements.dart';
 import '../compiler.dart';
 import '../constants/values.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
+import '../elements/names.dart';
 import '../elements/types.dart';
 import '../options.dart';
 import '../world.dart';
 import '../universe/world_builder.dart';
 import '../util/emptyset.dart';
 import 'constant_handler_javascript.dart';
 
 abstract class MirrorsData {
   /// True if a call to preserveMetadataMarker has been seen.  This means that
   /// metadata must be retained for dart:mirrors to work correctly.
@@ skipping 667 matching lines @@
   }
 
   /// Called when `const Symbol(name)` is seen.
   void registerConstSymbol(String name) {
     symbolsUsed.add(name);
     if (name.endsWith('=')) {
       symbolsUsed.add(name.substring(0, name.length - 1));
     }
   }
 }